fix(insights): ask the empty-state Coach question in the reader's language

A metric page without data offers to ask the Coach about the metric.
44 of the 49 pages passed that question as an English literal, so a
German, French or Korean reader got an English first message; only 5
read it from the catalog. 31 of the 44 were one sentence with the
metric's name swapped in.

`HealthKitMetricPage` now asks that sentence once, translated, around
the page's own title, and the 31 pages drop the prop. The 13 pages
with a question of their own (steps, HRV, sleep, mood, workouts, blood
pressure and others) read it from the catalog like the 5 did, in all
ten languages.

src/components/insights/__tests__/healthkit-metric-page-coach-prefill.test.tsx

@@ -0,0 +1,74 @@
+import { describe, expect, it, vi } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+import { I18nProvider } from "@/lib/i18n/context";
+import { primeMessages } from "@/lib/i18n/load-locale";
+import deMessages from "../../../../messages/de.json";
+
+primeMessages("de", deMessages);
+
+/**
+ * A metric page with no data offers the Coach a question in the reader's
+ * language. The pages used to pass their own English sentence, and most of
+ * them were one sentence with the metric's name swapped in; the page now asks
+ * that one, translated, with its own title, and a metric with a question of
+ * its own reads it from the catalog too.
+ */
+
+const prefills: Array<string | null | undefined> = [];
+vi.mock("@/components/insights/metric-empty-state", () => ({
+  MetricEmptyState: ({ coachPrefill }: { coachPrefill?: string | null }) => {
+    prefills.push(coachPrefill);
+    return null;
+  },
+}));
+vi.mock("@/hooks/use-ai-capability", () => ({
+  useAiCapability: () => ({ available: true, reason: null }),
+}));
+vi.mock("next/dynamic", () => ({ default: () => () => null }));
+vi.mock("next/navigation", () => ({
+  usePathname: () => "/insights/visceral-fat",
+}));
+vi.mock("@/hooks/use-auth", () => ({
+  useAuth: () => ({
+    user: { timezone: "UTC", dateOfBirth: null, gender: null },
+    isAuthenticated: true,
+  }),
+}));
+vi.mock("@/hooks/use-insights-layout-prefs", () => ({
+  useInsightsLayoutPrefs: () => ({ layout: null, compareBaseline: false }),
+}));
+vi.mock("@/hooks/use-insights-analytics", () => ({
+  useInsightsAnalytics: () => ({ data: { summaries: {} }, isEmpty: true }),
+}));
+
+import InsightsViszeralfettPage from "@/app/insights/visceral-fat/page";
+import InsightsStepsPage from "@/app/insights/steps/page";
+
+function emptyStatePrefill(node: React.ReactNode, locale: "en" | "de") {
+  prefills.length = 0;
+  renderToStaticMarkup(
+    <QueryClientProvider client={new QueryClient()}>
+      <I18nProvider initialLocale={locale}>{node}</I18nProvider>
+    </QueryClientProvider>,
+  );
+  return prefills.at(-1);
+}
+
+describe("the empty metric page's Coach question", () => {
+  it("is the shared sentence with the page's title, in the reader's language", () => {
+    expect(emptyStatePrefill(<InsightsViszeralfettPage />, "de")).toBe(
+      "Viszeralfett: Ich habe hier noch nichts erfasst. Was sagt dieser Wert über meine Gesundheit aus, und wie verbessere ich ihn?",
+    );
+    expect(emptyStatePrefill(<InsightsViszeralfettPage />, "en")).toBe(
+      "Visceral Fat: I haven't logged anything here yet. What does this metric tell me about my health, and how do I improve it?",
+    );
+  });
+
+  it("keeps a metric's own question, translated", () => {
+    expect(emptyStatePrefill(<InsightsStepsPage />, "de")).toBe(
+      "Ich habe noch keine Schritte erfasst. Was ist ein sinnvolles tägliches Schrittziel, und wie hilft mehr Gehen meiner Gesundheit?",
+    );
+  });
+});

src/components/insights/healthkit-metric-page.tsx

@@ -148,9 +148,8 @@ export interface HealthKitMetricPageProps {
    */
   statIcon?: ComponentType<{ className?: string }>;
   /**
-   * Optional Coach prefill for the empty-state launch. Falls back to a
-   * generic onboarding prompt threaded through `<CoachLaunchButton>`'s
-   * default.
+   * The empty state's Coach question, for a metric that asks its own. Without
+   * it the page asks the shared, translated one with its own title.
    */
   coachPrefill?: string;
   /**
@@ -223,6 +222,17 @@ export interface HealthKitMetricPageProps {
   afterAssessment?: ReactNode;
 }
 
+/**
+ * The empty state's Coach question for a metric that asks none of its own:
+ * one translated sentence around the page's title.
+ */
+function sharedCoachPrefill(
+  t: ReturnType<typeof useTranslations>["t"],
+  metric: string,
+): string {
+  return t("insights.subPage.emptyCoachPrefill", { metric });
+}
+
 export function HealthKitMetricPage({
   measurementType,
   fallbackMeasurementType,
@@ -424,7 +434,7 @@ export function HealthKitMetricPage({
           title={t(`${i18nPrefix}.emptyState.title`)}
           description={t(`${i18nPrefix}.emptyState.description`)}
           cta={ctaNode}
-          coachPrefill={coachPrefill ?? null}
+          coachPrefill={coachPrefill ?? sharedCoachPrefill(t, title)}
         />
       </SubPageShell>
     );